This invention relates to the testing of specimens of polymer material and more particularly to the determination of high frequency dilatational and shear velocities, storage and loss moduli and attenuation coefficients for polymer materials.
Ultrasonic testing of polymer materials is conventionally carried out by a buffer-rod technique, such as that described in J. R. Asay, D. L. Lamberson, and A. H. Guenther, J. Appl. Phys., 40, 1768 (1969); J. Williams and J. Lamb, J. Acoust. Soc. Am., 30, 308 (1958); and G. W. Paddison, Polym. Eng. & Sci., 14, 382 (1974). While accurate, the technique does not provide a complete characterization (except for transition temperature) because of the wave-propagation mode. Due to the high coupling losses in shear-wave transducers, with this technique the only wave mode that can be propagated in polymers at temperatures above the .alpha.-transition is the dilatational mode, and the dilatational modulus M from this mode is a composite of the bulk K and shear G moduli. A measurement in another wave mode is required in order to determine the individual moduli of the polymers. (As used herein, polymer materials include polymers and polymeric composites).
Ultrasonic immersion apparatus for measuring both the dilatational and shear velocities in solid specimens is known. A system for rotating the specimens to exceed the critical angle is described in H. A. Waterman, Kolloid-Zeitschrift and Zeitschrift Fur Polymere, 192, 1 (1963). This technique can be used for measurements on metals and room temperature measurements of polymers. However, because polymers become rubbery and can be deformed above the glass transition temperature, such an apparatus cannot be used for temperature dependent measurements at elevated temperatures. The rotation of the specimens by the apparatus is unacceptable above the glass transition temperature because the specimens may be deformed.
U.S. Pat. No. 3,858,437 Jarzynski et al. shows an apparatus in which the transducers are rotated about a stationary specimen. The present invention relates to improvements in this type of apparatus which make it possible to measure the high frequency storage and loss moduli for polymer materials.
The ultrasonic measurement technique described in the aforementioned Jarzynski et al. patent and in the articles B. Hartmann, and J. Jarzynski, J. Acoust. Soc. Am., 56, 1469 (1974); and B. Hartmann, and J. Jarzynski, Naval Ordnance Laboratory TR72-73, (1973) is a so-called "slope" measurement technique. The measurements made with Jarzynski et al. apparatus were limited to a narrow range of temperatures. In this technique different specimens having substantial differences in thickness are acoustically tested. Such a technique cannot be used on polymer specimens at temperatures above the glass transition temperature or at high frequencies. Very thin specimens of polymer material must be used in wide-range temperature dependent acoustic testing. So much energy is absorbed in the specimen that thick samples absorb all of the acoustic energy. Since it is necessary to determine the acoustic insertion loss property above the glass transition temperature, thin specimens which do not absorb all of the acoustic energy must be used. Also, in measurements such as those carried out by Jarzynski et al., the mass density of the specimen is measured at room temperature and it is assumed to remain constant throughout the measurements. This is not a valid assumption for polymers which have a mass density with a significant dependence upon temperature. Since the storage moduli of polymers are related to mass density, which is dependent upon temperature, provision must be made for measuring mass density at each of the temperatures at which acoustic measurements are made. Furthermore, the same polymer material may have a different mass density from specimen to specimen depending upon the rate of heating the specimens. Some polymers have atomic sized air voids which expand at different rates depending upon the rate of heating or cooling. Even if the specimens are heated at a constant rate, there is a geometry effect by which mass density at a particular temperature may depend on the shape of the specimen.
Another physical characteristic of polymers which is temperature dependent is specimen thickness. The specimen thickness is a parameter in the determination of storage and loss moduli. It may vary considerably if measurements are made over a wide range of temperatures.
It is an object of the present invention to provide an automated, ultrasonic immersion apparatus and method that measures dilatational and shear velocity, storage and loss moduli, and attenuation over a wide temperature range.